<template>
    <ul class="list">
        <li class="item" v-for="item of letters" :key="item"
               :ref="item"
               @click="handleLetterClick" 
               @touchstart.prevent="handleTouchStart"
               @touchmove="handleTouchMove"
               @touchend="handleTouchEnd"
        >{{item}}</li>
    </ul>

</template>
<script>
    export default {
        name: 'CityAlphabet',
        props: {
            cities: Object
        },
        computed: {
             letters() {
                const letters = []
                for (let i in this.cities ) {
                    letters.push(i)
                }
                return letters
             }
        },

        data () {
             return {
                touchstatus: false,
                startY: 0,
                timer: null
             }
        },
       updated () {
       this.startY = this.$refs['A'][0].offsetTop
       },

        methods: {
          handleLetterClick (e) {
            this.$emit('change', e.target.innerText)
          },
          handleTouchStart () {
             this.touchstatus = true
          },
          handleTouchMove (e) {
           if(this.touchstatus) {
             if(this.timer) {
                clearTimeout(this.timer)
             }
             this.timer = setTimeout(() => {
             
               const touchY = e.touches[0].clientY - 79
              const index = Math.floor(touchY - this.startY) / 23
             
             if(index >= 0 && index < this.letters.length){
                 this.$emit('change', this.letters[index])
             }
             },16)
           }
          },
          handleTouchEnd () {
             this.touchstatus = false
          }
        }
    }
</script>
<style scoped>
.list {
    /*垂直居中*/
    display: flex;
    flex-direction: column;
    justify-content: center;

    position: absolute;
    top: 1.58rem;
    right: 0;
    bottom: 0;
    width: .4rem;
}

.item {
    line-height: .45rem;
    text-align: center;
    color: #4682B4;

}

    
</style>